The present invention relates to expanded foam articles from synthetic thermoplastic materials and particularly to pre-expanding beads of such thermoplastic materials for use in subsequent molding processes.
In the process of the prior art, it is conventional to initially pre-expand the thermoplastic beads to form so-called "pre-puffs." These pre-puffs are expanded to at least fifty percent of volume of the final molded article. By preforming the beads, the mold can be substantially filled, thereby permitting a more uniform expansion of the beads, the elimination of voids, lower molding pressures, higher production cycles, and the formation of low density foam. The basic process is described by G. R. Franson, Plastics Technology, July, 1956, pp. 452-455.
Many pre-expansion techniques have been developed. Initially, the particular technique was not considered important, perhaps because production rates were less critical. Accordingly, U.S. Pat. No. 3,015,851 simply stated that one could use hot water, steam, dielectric heating, infra-red radiation or hot air. For economic reasons, steam was regarded as the heating medium of choice as noted in U.S. Pat. No. 3,086,248, but later direct steam heating fell into disfavor because the residual moisture had to be removed prior to molding to prevent irregular expansion and the crushing of the beads. Often, a storage period of 24 hours was required to normalize the pre-puffs prior to molding.
U.S. Pat. No. 3,252,228 turned to pre-expanding in a moisture-free atmosphere; however, these techniques, too, were not without their problems. Free flowing, uniform pre-puffs became more imperative as still higher production rates were demanded. The aforesaid U.S. Pat. No. 3,252,228 and U.S. Pat. No. 3,821,342 show exotic means of fluidizing the pre-puffs to obtain uniformity and dry free-flowing particles. U.S. Pat. No. 3,577,360 shows the use of an agitator to keep the particles moving to prevent agglomeration.
As the art progressed, it was recognized that appropriate pre-expansion was an art far more complex than originally understood. Boiling the beads in hot water for an arbitrary period or merely heating them in hot air or steam until some predetermined density was achieved was not sufficient for producing uniform, dimensionally stable products at ever faster production rates. It was soon recognized that the type of polymer, the volatile content of the bead before and after pre-expansion, and the moisture in the environment must all be considered. And finally, the equipment for pre-expanding and molding the foam has to be capable of high speed and trouble-free operation.
In addition to the patents noted above, pre-expansion is shown in U.S. Pat. Nos. 2,944,292 and 3,494,600 (hot water); U.S. Pat. Nos. 3,139,272 and 3,973,884 (steam); U.S. Pat. Nos. 3,273,873; 3,278,658; 4,032,609 and 4,060,354 (dry air); U.S. Pat. Nos. 3,023,175 and 3,378,245 (steam and hot air); U.S. Pat. Nos. 3,104,424 (radio frequency); and U.S. Pat. No. 3,262,682 (conduction).
U.S. Pat. No. 3,577,360 shows a process for pre-expansion wherein the beads are first heated in a closed vessel and then evacuated to pre-expand and remove a substantial amount of the blowing agent. It is disclosed that the atmosphere should be substantially dry. No details of molding are shown and water is added as a coolant. This process, however, does not produce stable pre-puffs and requires prolonged evacuation periods.